1. Field of the Invention
This invention is directed to the use of coated free flowing sand comprising a curable phenolic resin system, useful as a subterranean proppant. The improved resin system of this invention is a phenol-formaldehyde resin which may contain alkoxyl or aroxylphenol, alkylphenol or arylphenol and is not cured by hexamethylenetetramine. The invention also relates to the use of a mixture of resole and novolak resins without alkylation, arylation or other modifiers to coat sand particles.
2. Description of Related Art
The use of resin-coated proppants or propping agents is increasingly important in treating subterranean formations. One of the primary uses of curable resin-coated sand is to minimize or prevent proppant flowback from a fracture during cleanup or production in oil and gas wells. Resin-coated curable proppants are also useful in sand control. In hydraulic fracturing, coated particles are used to maintain the fracture in a propped condition, and to improve the stability of proppants at high closure stresses. Sand or other substrates have been coated with resins such as an epoxy, furan, or phenolic resin. However, these systems, especially partially cured and curable resin-coated proppants, can hinder the interaction of the breaker with fracturing hydraulic fluids, causing delay in fluid breaking and resulting in various production difficulties. It has also been determined that hexamethylenetetramine, which is often used to initiate the cure of phenolic resins, can be the cause of the delays in fracturing fluid breaking. The production difficulties can include, for example, the cross-linked gel not reducing in viscosity a required so the fluid can flow back out, thus delaying production from the well. Furthermore, if the fracturing fluid gel does not break and allow the curable resin-coated proppants to come together before the resin cures, they cannot bond together forming a three dimensional matrix. If they do not bond together, flowback may occur. It is also desirable to have the gel which is formed break at some designated time to reduce viscosity and allow water and ultimately gas or oil to flow out of the well. Also contributing to these problems are water soluble components, such as unreacted phenol or low molecular weight resin components, often present in the phenolic resins.
The most widespread fracturing treatment to achieve proppant consolidation is the use of curable phenolic resins pre-coated on the proppant. Generally, such coated materials are manufactured away from the well site and delivered to the well location. Another approach is to coat the proppant on-site by adding required resin materials to the fluid so that coating occurs while the proppant is being pumped or even after the sand is pumped downhole.
Rankin, et al., U.S. Pat. No. 2,675,335 issued Apr. 13, 1954, relates to thermosetting phenol-formaldehyde compositions useful for the manufacture of laminated articles with desirable electrical properties. Rankin et al. teaches the addition to the composition of minor amounts of monobutyl phenol and dibutyl phenol. Rankin et al. teaches that the resulting laminating resins have better water resistance, retention of electrical properties, and improved flow characteristics enhancing the penetration of the resin into paper and fabric. Use of the claimed materials as a subterranean proppant is not suggested in Rankin et al., and hexamethylenetetramine is used as a catalyst.
Hanauye et al., U.S. Pat. No. 4,179,429 issued Dec. 18, 1979, relates to a phenol resin useful as a varnish for impregnating a base to thereby obtain superior mechanical and electrical characteristics. Phenol resins containing as the phenol component thereof nonylphenol are identified for improving punching quality in electrical laminates. Subterranean proppants are not suggested.
Holik, U.S. Pat. No. 4,317,896 issued Mar. 2, 1982, teaches a binder for foundry aggregate comprising a butylated phenol formaldehyde resin, plus a resole resin, plus a novolak resin. Holik is not directed to subterranean proppants and provides a resin which is cured with a diisocyanate and a tertiary amine.
Saeki, et al., U.S. Pat. No. 4,426,484 issued Jan. 17, 1984, teaches acceleration of the cure of a resole-type phenol aldehyde resin by the addition of resorcinol. The resin comprises a mixture of resole-type and novolak-type resins.
Okada et al., U.S. Pat. No. 4,873,145 issued Oct. 10, 1989, teaches a water resistant resin-coated aggregate for use in concrete with a novolak resin. However, Okada et al. cures the resin by use of hexamethylenetetramine, and does not suggest subterranean proppant applications.
Graham, et al., U.S. Pat. No. 4,888,240 issued Dec. 19, 1989, teaches an improved resin for use as a proppant in hydraulic fracturing of subterranean formations. Alkylphenol modification of the resin is not discussed and Graham et al. uses hexamethylenetetramine as both a catalyst and a source of formaldehyde.
Whitehurst, et al., U.S. Pat. No. 4,969,522 issued Nov. 13, 1990, teaches the preparation of a polymer-coated support in subterranean oil well formations. However, the polymers of Whitehurst, et al. are olefins cured by traditional Ziegler catalysis. Alkylphenol modified phenol-formaldehyde resins are not suggested.
The applicants and others have utilized mixtures of resole resin and novolak resin curable with hexamethylenetetramine in subterranean formations, where said resins are not arylphenol- or alkylphenol-modified.
Therefore, it is desirable to have a proppant for subterranean use which will have reduced interactions between the breaker and hydraulic fluid relative to those of curable proppants currently available. Thus, it is an object of the present invention to provide a resin useful as a coating for proppants, whereby the resin coating does not utilize hexamethylenetetramine in a quantity sufficient to effect the cure. It is another object of the present invention to reduce the water solubility of the phenolic resin on the coated sand, or leachate therefrom, to thereby minimize the deleterious interaction between the breaker and the hydraulic fluids.